By the renormalization procedure [1]}, we modify \(\nabla W\)  as follows
\(\sum _j \dfrac{m_j}{\rho _j}(\mathbf {r_j}-\mathbf {r}) \footnotesize {\bigotimes } L(\mathbf {r})\nabla W(|\mathbf {r}-\mathbf {r}_j|)=\begin{pmatrix}1 & 0 & 0\\0 & 1 & 0\\0 & 0 & 1\end{pmatrix},\) 
